Orientation of ferrites



Dec. 4, 1962 L. DAVIS, JR 3,067,140

ORIENTATION OF FERRITES Filed June 16, 1959 2 s 3 7 l l 5 v6 l a 1 V I l I I 5 F G I F G. 2

.2'6 morp CONTROL VOLTAGE EXC/MTION SOURCE FIG a wvmronuifiajfi DAy/s; um v far {K United States Patent Ofilice 3,067,140 Patented Dec. 4, 1962 3,067,140 ORIENTATION F FERRITES Luther Davis, .Ir., Wayland, Mass., assignor to Raytheon Company, Lexington, Mass., a corporation of Delaware Filed June 16, 1959, Ser. No. 820,788

14 Claims. (Cl. 252--62.5)

This invention relates generally to ferromagnetic, ferroelectric, and ferrite materials, and more particularly to a method of orienting the particles of these materials into a predetermined fixed disposition, and to the resulting structures attained. For the purposes of this application, the slight difference which technically exists between ferromagnetic and ferrimagnetic materials is to be disregarded, and it is to be understood that ferrimagnetiomaterials are included within the purview of the present invention. As used in this specification, the word ferrite'f is intended to include ferromagnetic materials, although it is realized that technically only ferrimagnetic materials are classed as ferritcs.

Ferritcs have been used extensively in microwave. devices, and are assuming increasingly greater importance for use in microwave systems utilizing the microwave region of the electro-magnetic spectrum. It is well known to those skilled in the art that in crystalline ferrite structures, the magnetization tends to be directed along certain definite crystallographic axes designated as the easy axes of magnetization. The directions of these easy axes of magnetization depend upon the anisotropy constants of the material. I

It has been found that some ferrite devices for use in microwave applications, such as isolators, for example, provide better operation if the easy axes of magnetization of each of the crystals which compose the material are, respectively, aligned or polarized parallel to each other. Alignment of these axes provides a material with a narrower resonance width, which usually represents an advantage in microwave system applications.

Despite the fact that it is highly desirable to obtain ferrite materials, all of whose easy axes are aligned, the

materials which have heretofore been used in microwave devices have generally been composed of randomly dis-i persed crystals. the easy axes of magnetization of which are not aligned. This has been primarily due to the fact that up to the present time there has been no satisfactory method for producing such an orientation except in thc very limited case of one dimensional alignment along only In accordance with the present inycntron, there is provided a method and resulting structure a single easy axis.

for orienting ferrite materials. ferromagnetic materials,

or ferroelectric materials. along more than one dimension Briefly, the present invention involves subjecting ferr-o magnetic or ferroelectric powders to a'strong orienting field, a magnetic field in the case of ferromagnetic material and an electric field in the case of fcrroelectric material. In one embodiment, the powder may be suspeadcd ih a suitable medium, for example. an epoxy resin or othcr suitable polymer and then subjected to the field during polymerization of the suspending medium in order to orient the crystals. The entire body of the material. ferrite powder and suspending medium. is consecutively moved through an angle which successively aligns each of "the easy axes of polarization or magnetization parallel with the direction of the applied field. The time requiredto move through this angle should he very short compared to the time spent in the stationary po itions corresponding to the easy axes directions, so that, in effect, the material sponding to third and fourth directionsjwhos geometrical is snapped rapidly from stationary position to stationary position. At some time during this process, the viscosity of the suspension will have increased to a value where the time required for thermal misorientation will be long compared to the positioning intervals, but not so high as to prevent orientation of the particles by the field which is being continuously applied. Upon complete solidification of the suspended medium, all of the embedded particles have a complete rnulti-dimensional spatial orientation in which all of the easy axes of the crystals are, respectively, aligned substantially parallel to each other.

The invention will be'hetter understood as the following description proceeds taken in conjunction with the accompanying drawing wherein:

FIG. 1 shows the directions of the easy axes of magnetization of a cubic single crystal of ferrite material having a negative anisotropy constant:

a positive anisotropy constant; and

FIG. 3 shows an exemplary system which can be utilized according to the invention for orienting ferrite material along all of its easy axes of magnetization.

For a material composed of cubic crystals having a negative anisotropy constant, the easy axes of magnetization lie along the four body diagonals of the cube. Such a cubic structure is shown in FIG. 1 in which the crystal is designated at it), while the easy axes of magnetization lie along the diagonals 1, 2, 3 and 4. Nickel ferrite, nickel aluminate ferrite, and manganese ferrite are'examples of. ma erials of this type.

In FIG. 2 there is shown a cubic crystalline ferrite structure which has a positive anisotropy constant. The cubic crystal 5 has its easy axes of magnetization lying along a direction parallel to the three cube edges 6, 7 and S. Fcrrites containing an appreciable amount of cohalt are examples of materials of this type.

Referring now to FIG. 3, there is shown an embodiment of the method of orienting ferrite crystal particles having a negative anisotropy constant, such 'asthe cube shown in FIG. 1, which is accomplished by suspending the particles 'in a suitable viscous medium, which may be, for example. an epoxy resin such as I-Iysol 6020, manufactured by the Houghton Laboratories, Inc., Clean, N.Y. In the case of a ferromagnetic material. a magnetic field is continuously applied to the suspension along a direction corresponding to the direction of one of the easy axes of -magncti z ation, the direction of field application being progressively changed in stepped movements. If the material to be oriented comprises a ferroelectric. material, an electrie'field would be continuously applied along the di rection of the respective easy axcsof polarization. After the field has been applied along a first direct-ion corresponding to a first easy axis for a sufiicient t'irne period, say, oh the order of ten seconds, along a first direction compared to the time during which thetest tube .20 is snapped through the angle nccessary to, align a. next sitecessive easy axes of polarization. For the *cnystalof FIG. 1, rotated about an axis normal. to, a crystal face, this single would be substantially In mshnilar manner; the rnaten'al is consecrrrivcly-steppedto. positions correrelationships to the first and second directions correspond to the geometrical relationships that exist between body diagonals 3 and 4, and body diagonals 1 and 2. The continuous application of a magnetic field along each axis, together with the stepped movement from stationary position to stationary position is repeated all during the time 5 that the viscosity of the material is increasing preferably to the point where the suspension solidifies into a solid body. The field strength used is preferably sufficient to magnetize the individual particles to saturation, i.e., as large as the anisotropy field and the saturation magnetization for the material being oriented. In the embodiment described above, the field strength may be on the order of 5,000 gauss.

If all of the particles were approximately spherical, there would be a substantially complete alignment of the crystal axes of the particles if orientation along two easy axes was obtained since the third must automatically fall in place. However, since the general shape of known particles of ferrite material is not spherical, a more perfect orientation is achieved by orienting all three easy axes, or more as the case may be, which procedure averages out the efiect of shape anisotropy. The detrimental effects of a non-spherical shape of the particles can be minimized by making the time of movement through the axis angle very short. Throughout the process of continuously applying the magnetic field, the medium in which the ferrite particles are suspended is allowed to harden and, hence, its viscosity is increased. As the orientation process continues, the freedom of motion of the crystals becomes more and more restrained so that at the end of the process, the crystals are essentially frozen in a state such that substantially complete alignment of the respective crystal axes is attained.

The application of a continuous field along an easy axis of magnetization may be achieved in many ways, one of which is shown in FIG. 3. In this figure, a test tube 20 containing particles of a ferromagnetic material suspended within a medium 21 is positioned in the field of a coil 22 such that the direction of the magnetic field produced by a current through the coil lies along a direction correspond ing to one of the body diagonals of the crystalline particles. The suspending medium may be a plastic, such as Hysol 6020, previously identified. The particles are single crystal with cubic crystal structure, and of arbitrary shape having a negative anisotropy constant. Coil 22 is arranged so that the direction of the magnetic field (as shown by the dashed arrow 26) produced by a current through the coil lies at an angle at substantially 5.4.4 with respect to the longitudinal direction of the test tube 20. This rela-; tionship between the longitudinal axis of the test tube and the direction of the applied field assures that, as the tube is stepped about its longitudinal axis, the field will be applied along the correct direction corresponding to the body diagonals. Coil 22 is connected to an appropriate source of excitation voltage 23.

Initially, the test tube 20 is connected to an actuating source operative to intermittently snap, the test tube hrough he desired aligning angle by a mechanism comprisingaf first gear 27 attached to the test tube 20. anddri-ven'by a, second gear 28 secured to the shaft 29 of a motor'3'S. The motor 35 is connected to a sburce=of ni'o'tor control voltage and includes appropriate mechanism forsnappingth test tube through the desired sol'rd'angle. *For exf' ple well-known escapernent mechanisms similarjto those 'hse'df in clocks and watches may be used to inter'rn tenth cause the tube 20 to be stepped. The intermittent rotation of shaft 29 causes the test tube to be moved about its longitudinal axis in the-direction of therarrro'w 2,5.

tion to a second stationary position", test tu-be 20 is rotated. 7

diagonal. Accordingly;orientation along-this, axis will with each of the appropriate'crystal axes.

60 anisotropy constant. I

take place. Likewise, when the test tube is stepped to positions which are 180 and 270 removed from its initial position, orientation along these axes will take place.

This process is cyclically repeated during the polymerization of the suspension 21 until the suspension has hardened. However, it is believed that substantially complete orientation is achieved before the suspension has hardened completely so that it is possible to decrease the hardening time by adding a suitable accelerator, such as 10 Hysol Hardener C, which is manufactured and sold by the Houghton Laboratories, Inc., Olean, N.Y.

Alternatively, the suspension may be maintained in a fixed position and the coil 22 may be intermittently moved so that the field produced by it is sequentially aligned Any method which provides a relative motion between the suspension and the coils is suitable.

In addition, the suspension of the ferrite particles need not necessarily be limited to suspension in a plastic me- 20 dium, and further need not necessarily be suspended in another material at all. As an alternative, the crystal particles may be placed in a test tube and their freedom of motion may be controlled by a pressure device, such as a piston, inserted into the test tube. The particles in the test tube may then be ultrasonically vibrated so that the particles are given substantial freedom of motion. equivalent to the freedom of motion enjoyed by the particles in the plastic fiuid at the start of the previouslydescribed process. As the process continues and the test tube is moved in a stepped manner, the vibration amplitude may be gradually reduced and the pressure exerted by the piston gradually increased so that the motion of the crystal particles is gradually restrained during the process. Thus, at the end of the process, the closelypacked particles are formed into a body in which the easy axes of magnetization are correctly aligned, but in which the particles are not suspended in a plastic medium, as previously described. Alternatively, in the case of the suspended embodiment, the suspending medium may be 40 removed, and the orientedferrite powder sintered into to perform the process at other temperatunes depending upon the viscosity and rate of hardening of the plastic medium involved.

In the embodiment in which Hysol was used in the plastic medium, the viscosity'of the sus pension at the start of the,process was of the order of 50, -100 poises, with substantially complete orientation being achieved when the visco-s-ityreached a. value considerably shorto'f the substantially infinitevalue d'fviscosity which exists when the suspension completely hardened. -Al'th-ough ther-process of the invention has been par- 'ticularly d'es cr'ibed with respect to the orientation of crystals having a negative anisotropy constant, it will be obvious to those skilled in the art'thl' h applicable to the orientationof'crys't the latte Similarly, certain-hexagonal crystals ha g subiec't'eddo th process-of the present invention It can-'thufs'beseen that there has been"pro'vi ded multi- -'difr nrrsionral ly orientedfferrite material, which may be used to advantage in transformer cores, storage elements,

it wasimpossible to realize the advanmes rif'ferijitesin the eQes along the preferred axes 'tzhereby yieldingflte on f the-field will; accordingly,j-.

ers'jof mag'neti-zationj rnay also be and 'nzta'gnefic'ar'rrpllifiers,- for example, In. the" case of a; ferrite whicnwas oriented in only one dirnenfs ion, the. brittle material -'could'not be wound in tape form so that I e processjs also superior characteristics obtained with tape-wound cores. in addition, the use of multi-dimensionally oriented ferrite material eliminates the microwave resonance line broadening caused by the magnetocrystalline anistropy of randomly oriented powders.

Although there have been described what are considered to be preferred embodiments of the present invention, various adaptations and modifications thereof may be made without departing from the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

l. A crystalline ferrite material comprising a plurality of minute particles suspended in a non-magnetic medium and having a plurality of crystallographic easy axes, at

least one of said axes lying in a plane other than that formed by at least two other of said axes, having multidimensional alignment of said crystallographic easy axes formed by subjecting said material to a magnetic field, sequentially changing the direction of said magnetic field said field is being applied to solidify said material into a compact mass.

6. The method of orienting a ferrite material having a plurality of crystallographic easy axes, at least one of said axes lying in a plane other than that formed by at least two other of said axes, said method comprising sub jecting said material to a magnetic field directed along one of said easy axes of said material, sequentially applying said field along a different crystallographic easy axes of said material and gradually restraining the freedom of motion of said material while said field is being applied to solidify said material into a compact mass.

7. The method of orienting the crystals comprising a ferrite material having a first plurality of crystallographic easy axes, at least one of said axes lying in a plane other than that formed by at least two other of said axes, said method comprising suspending said crystals in a nonmagnetic medium, applying a magnetic field to said suspension to align said crystals along a second plurality with respectto said material to align a second of said of Said Crystallographic y 3X68 0f Orientation, and

crystallographic easy axes of the particles comprising said material and gradually restraining the freedom of motion of said material while said magnetic field is being applied to solidify said material into a compact mass.

2. A ferrite body comprising a plurality of minute particles suspended in a non-magnetic medium and having a first plurality of crystallographic easy axes, at least one of said axes lying in a plane other than that formed by at least two other of said axes oriented along a second plurality of said crystallographic easy axes formed by subjecting said material to a magnetic field directed along one of said easy axes of said material, sequentially applying said field along a different crystallographic easy axis of said material and gradually restraining the freedom of motion of said material while said magnetic field is being applied to solidify the material into a compact mass.

3. A crystalline ferrite body comprising a plurality of minute particles suspended in a non-magnetic medium and having a first plurality of crystallographic easy axes,

"at least one of said axes lying in a plane other than that formed by at least two other of said easy axes, and having its individual particles oriented with a second plurality of said respective crystallographic easy axes in a substantially parallel alignment formed by subjecting said material to a magnetic field sequentially changing the direction of said magnetic field with respect to said material to align a second plurality of said crystallographic easy axes of the particles comprising said material and gladually restraining the freedom of motion of said material while said magnetic field is being applied to solidifyithe material into a compact mass.

4. A polycrystalline ferriate body comprising a plurality of minute particles suspended in a non-magnetic medium and having a first plurality of crystallographic easy axes, at least one of said axes lying in a plane other material while said magnetic field is being applied to solidify said material into a compact mass.

5. Thermethod of orienting a ferrite material havinga first plurality of crystallographic easy axes, at least one. of said axes lying in a plane other that formed by" material to a magn'etictfield sequentially changing the direction of said field with respect to said material to align a second plurality of said crystallographic easy axes of the crystals comprising said material andigra'dually gradually restraining the freedom of motion of said crystals in said suspension.

8. The method of orienting the crystals comprising a ferrite material having a first plurality of crystallographic easy axes, at least one of said axes lying in a plane other than that formed by at least two other of said axes, said method comprising suspending said crystals in a nonmagnetic medium, and sequentially applying a continuous magnetic field to said suspension to align said crystals along a second plurality of said crystallographic easy axes of magnetization while gradually restraining the freedom of motion of said crystals in said suspension.

9. The method of orienting ferromagnetic crystals having a first plurality of craystallographic easy axes, at least one of said axes lying in a plane other than that formed by at least two other of said axes, said method comprising applying a magnetic field to said crystals, and sequentially moving said crystals with respect to the direction of said magnetic field to align said crystals with a second plurality of said crystallographic easy axes of magnetization substantially parallel with each other while causing said crystals to compact into a solid mass.

10. The method of orienting the crystals of a ferrite body having a first plurality of crystallographic easy axes, at least one of said axes lying in a plane other than that 0 formed by at least two other of said axes, said method comprising suspending said crystals in a plastic non-magnetic medium that initially provides freedom of motion for said crystals, subjacting said suspension to-a continuous magnetic field, and relatively moving said suspension with respect to said field in order to polarize said crystals along a second plurality of said crystallographic easy axes of magnetization white progressively restraining the freedom of motion of said crystals to form a compact body.

11. The method of orienting a body of ferromagnetic 1 material having a plurality of crystallographic easy axes, at least one of saicl axes lying in a plane other than that formed by at least two other of said axes, said method comprising suspending said crystals in a plastic non-mag- (lb netic medium, and appl ing a continuous magnetic field to said medium while said rfiedium is snapped to different positions in said field, and causing said plastic medium to gradually harden.

l2. Them'ethod of orienting a ferrite body composed of-substantially cubic crystals (herein the crystallographic easy axes of-magnetization correspond to the bod diagonals of said .crystals, said method comprising f suspending said crystals in a plastic non-magnetic medium, sub'ectin said suspension in'a first osition to a at least two other of said ax:es,compr1smg sub ecting said J g p continuous magnetic'field to align said crystals along one of said easy axes, moving said suspension to a different crystallographic easy axis position in said field during a ttmejnterval which is short compared to the time interval'spent i'n-said fitist position while gradually causing restraining the freed'onroif motion of said material while {(5 said suspension to harden.

13. The method of orienting the crystals comprising a ferrite material having a first plurality of crystallographic easy axes, at least one of said axes lying in a plane other than that formed by at least two other of said axes, said method comprising suspending said crystals in a nonmagnetic medium, applying a continuous magnetic field to said suspension to align said crystals along a second plurality of said crystallographic easy axes of magnetization while gradually restraining the freedom of motion of said crystals in said suspension, and heating said suspension to remove said medium, whereby said crystals are sintered into a ceramic mass.

14. The method of orienting the ferromagnetic crystals having a first plurality of crystallographic easy axes, at least one of said axes lying in a plane other than that formed by at least two other of said axes, said method comprising suspending said crystals in a non-magnetic medium, applying a continuous magnetic field to said suspension to align said crystals along a second plurality of said crystallographic easy axes of magnetization while gradually restraining the freedom of motion of said crystals in said suspension, and heating said suspension to remove said medium, whereby said crystals are sintered into a ceramic mass.

References Cited in the file of this patent UNITED STATES PATENTS 2,532,876 Asche et al. Dec. 5, 1951 2,687,500 Jones et al Aug. 24, 195 2,762,778 Gorter et al. Sept. 11, 1956 2,827,437 Rathenau' Mar. 18, 1958 2,837,483 Hakker et a1 June 3, 1958 2,964,793 Blume Dec. 20, 1960 2,974,104 Paine et a1 Mar. 7, 1961 FOREIGN PATENTS 7 758,320 Great Britain Oct. 3, 1956 OTHER REFERENCES Morrill: General Electric Review, August 1950, pp. 16-21.

Stuijts et al.: Philips Technical Review, vol. 19, 1957/58, No. 7-8, pub. Feb. 10, 1958, pp. 209-217.

Watson'et al.: Journal of Applied Physics, vol. 29, No. 3, pp. 306-308, March 1958.

Popper: Journal of Electrical Engineers, vol. 2 (new series), No. 20, pp 450-457. 

1. A CRYSTALLINE FERRITE MATERIAL COMPRISING A PLURALITY OF MINUTE PARTICLES SUSPENDED IN A NON-MAGNETIC MEDIUM AND HAVING A PLURALITY OF CRYSTALLOGRAPHIC EASY AXES, AT LEAST ONE OF SAID AXES LYING IN A PLANE OTHER THAN THAT FORMED BY AT LEAST TWO OTHER OF SAID AXES, HAVING MULTIDIMENSIONAL ALIGNMENT OF SAID CRYSTALLOGRAPHIC EASY AXES FORMED BY SUBJECTING SAID MINERAL TO A MAGNETIC FIELD SEQUENTLLY CHANGING THE DIRECTION OF SAID MAGNETIC FIELD WITH RESPECT TO SAID MINERAL TO ALIGN A SECOND OF SAID CRYSTALLOGRAPHIC EASY AXES OF THE PARTICLES COMPRISING SAID MATERIAL AND GRADUALLY RESTRAINING THE FREEDOM OF MOTION OF SAID MATERIAL WHILE SAID MAGNETIC FIELD IS BEING APPLIED TO SOLIDIFY SAID MATERIAL INTO A COMPACT MASS. 